1. Field of the Invention
The field of the present invention is spectroscopy and in particular hyperspectral sensors.
2. Background
In a typical dispersive spectrometer, a scene is imaged onto a slit so that light from a thin portion of the scene passes through the slit. Light from the thin portion of the scene is then collimated, passed through a dispersive element such as a prism or grating, and imaged onto a focal plane. The resulting image on the focal plane is a spread spectrum image of the thin portion of the scene passed through the slit. Thus, the spectrum for each pixel of the image from the slit may be recorded by a focal plane array. Usually, the slit is scanned over the image of the scene to create what is commonly called a data cube in which the spectrum for each slit pixel in a two-dimensional scene is stored.
The data cubes created by the above process may be very large. For example, if a 256xc3x97256 focal plane array is used, for any given thin portion of an image, 256 spectral pixels are measured for each of the 256 slit pixels. If the slit is scanned across 256 separate thin portions of the scene, then the total data cube will consist of 256xc3x97256 slit pixels, and each slit pixel is associated with 256 spectral pixels. Altogether, this amounts to 16 million data points within the data cube. Assuming two bytes of information for each data point, such a data cube would contain approximately 32 Megabytes of data.
In order to image an entire industrial region using a dispersive spectrometer, very large data cubes are necessarily generated, with the data cubes containing many Gigabytes of data. The overall size of such large data cubes and the time required to process the data may be reduced by optimizing the spectral sampling.
The limiting size of currently available infrared focal plane arrays provides additional incentive to optimize the spectral sampling. For example, in the long-wavelength infrared (LWIR) band, current technology supports only about 256xc3x97256 pixel focal plane arrays suitable for high performance dispersive sensors where a 1024xc3x971024 pixel focal plane array would be more desirable to increase spatial coverage and provide better spectral resolution. However, not only are the currently available focal planes restricted in size, but they are also very expensive, frequently costing millions of dollars to develop.
Dispersive spectrometers using gratings typically produce a spectral sampling that is approximately uniform in wavelength. Thus, each spectral pixel samples an equal range of wavelength, such as every 10 nanometers. Prism based dispersive spectrometers, however, sample non-uniformly in wavelength according to the variation in refractive index for the particular material used.
Both of these dispersive designs do not match the ideal sampling for gaseous materials, which is constant sampling in wavenumber (1/wavelength) over a fairly broad spectrum. Many gaseous materials tend to have spectral line widths which are constant in wavenumber. Thus, when monitoring materials with such characteristics, it is often desirable to have sampling intervals that are approximately constant in wavenumber. However, for some applications it may be desirable to stress one portion of the band over another or to have a specific variation in spectral resolution across the band being analyzed.
The present invention is directed to a dispersive spectrometer that utilizes a grism to disperse light from a thin portion of a scene. The dispersive spectrometer comprises in optical alignment a primary lens, a slit, a collimating lens, a grism, and a focusing lens. The primary lens images a scene onto the slit and light from a thin portion of the scene passes through the slit. Light from the thin portion of the scene is thereafter dispersed by the grism. The grism includes a diffractive element integral to a surface of the grism. The grism is disposed so that light passing through the dispersive spectrometer has an angle of incidence upon the surface including the diffractive element that is greater than one-third of the critical angle at the surface. Additionally, the grism is oriented so that the diffractive element disperses light from the thin portion of the scene in a direction that is perpendicular to the major dimension of the thin portion of the scene. The focusing lens defines a focal plane upon which the dispersed light is imaged.
In a first separate aspect of the present invention, the grism may be formed of a material having an index of refraction that varies over a range of wavenumbers. In this case, the index of refraction of the grism may disperse light either in the same direction as or in a direction opposite from the dispersion provided by the diffractive element.
In a second separate aspect of the present invention, the dispersive spectrometer further comprises a focal plane array disposed at the focal plane. The focal plane array detects the dispersed light from the thin portion of the scene. The focal plane array may be used to detect light in specific bands.
In a third separate aspect of the present invention, the dispersive spectrometer further comprises a prism optically disposed between the collimating lens and the grism. The prism disperses light from the thin portion of the scene in either the same direction as or in a direction opposite from the dispersion provided by the diffractive element. The combination of a prism and a grism may be used to additionally shape the spectral output of the dispersive spectrometer as desired.
In a fourth separate aspect of the present invention, the dispersive spectrometer further comprises a second grism optically disposed between the collimating lens and the first grism. The second grism disperses light from the thin portion of the scene in either the same direction as or in a direction opposite from the dispersion provided by the diffractive element. The combination of the two grisms may be used to additionally shape the spectral output of the dispersive spectrometer as desired.
In a fifth separate aspect of the present invention, any of the foregoing aspects may be employed in combination.
Accordingly, it is an object of the present invention to provide an improved dispersive spectrometer. Other objects and advantages will appear hereinafter.